1. Field of the Invention
The invention relates to catalysed cationic paint binders which are water-thinable after protonation and contain bismuth salts of aliphatic hydroxycarboxylic acids, to a process for the preparation of these binders and to their use.
2. Description of Related Art
Cationic paint binders, employed in particular in the formulation of cathodic electrodeposition coatings, crosslink at elevated temperature to a significant extent by transesterification, transamidation, transurethanisation or by the reaction of terminal double bonds.
It is known that such crosslinking reactions are catalysed by metal compounds. Catalysis of this kind is necessary in virtually every case for the curing of cathodically deposited coating films, in order to achieve the profile of properties required by industrial users.
The most important of the catalysts conventionally used in current practice are lead compounds and tin compounds. However, the use of toxic or ecologically objectionable lead compounds or tin compounds is being increasingly made more difficult, and a ban on the use of such substances is to be expected.
There is therefore a particular need for physiologically and ecologically more acceptable catalysts which can be employed in cathodic electrodeposition coatings.
It has long been known that bismuth compounds catalyse the formation of urethane structures from isocyanate and hydroxyl groups (J. H. SAUNDERS and K. C. FRISCH, Polyurethanes, Chemistry and Technology, in High Polymers, Vol. XVI, Part I, Interscience Publishers, a divison of John Wiley and Sons, New York, Fourth Printing, July 1967, page 167).
Lists of metals suitable for use in electrode-position coatings also include bismuth, for example in EP-A2-138,193 and in EP-A1-0,264,834.
EP-A2-138,193 describes the use of the salts, specifically the acetates, of preferably divalent metals for improving the solubility of polymers.
In EP-A1-0,264,834, it is attempted to achieve a homogeneous distribution of metal salts or organometallic compounds in polymer microparticles either by an "impregnation" procedure, by polymerisation in the presence of the stated metal compounds or by copolymerisation of ethylenically unsaturated metal compounds.
The choice of bismuth compounds which can be used in electrodeposition coatings is in principle severely restricted. The more readily available salts of relatively long-chain acids, such as Bi octanoate or Bi neodecanoate, when employed in cationic binders, cause defects in the coating film due to oil-like exudations. Inorganic bismuth compounds are difficult to distribute by mixing into the binder or into a pigment paste, and in this form are of low catalytic activity.